Radio control circuit



Sept. 15, 1936- l., M. PERKINS RADIO CONTROL CIRCUIT Filed Aom. l14, 1932 bhw @I We mi www (l www bw .SSH

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Laurence M Fark/'175 non nunon m ggg N lf mm/MSMM Patented Sept. 15, 1936 UNITED STATES RADIO CONT Laurence M. Perkins, Eri assignments, to Radio New York, N.

Application October 14,

Claims.

This invention relates to a control circuit for a radio receiver and more particularly to an automatic output control and auxiliary manual controls to supplement the action of the automatic control.

In the ordinary automatic output circuits, where an auxiliary control tube is used, the control tube is connected across the detector tuned circuit in such a way that it adds capacity to that circuit, and prevents it from tracking with the other tuned circuits across the band of frequencies to be received. Such circuits, also, do not have a control for limiting the maximum sensitivity which the automatic control will cause the receiver to have when passing between stations in tuning, or when receiving a Very weak station. Hence, the noise at those times may be very great. Another feature which those circuits do not have, is that of automatically controlling the detector tube at the same time an intermediate or radio frequency amplifier tube is controlled.

An object of this invention is to provide a new improved automatic gain control for a radio receiver.

Another object of this inventionis to provide an automatic output control having a manual control for the maximum sensitivity which the automatic control can cause the receiver to reach. This feature provides a means for limiting the amount of noise picked up between stations as the receiver is tuned, as well as the noise received along with weak stations. This has been accomplished by providing a potential control for one of the elements of the control tube.

Another object of this invention is to provide an automatic output control having a manual control for the audio frequency stages to control the audio frequency output for a given detector output. This has been accomplished by providing the by-pass or voltage divider circuit between the detector output and audio frequency amplier.V

Another object of this invention is to provide an automatic volume control having a manual control for adjusting the volume level at which stations are to be received. This has been accomplished by providing a manual detector grid bias control and a manual grid bias control for the radio or intermediate frequency amplifier.

Another object of this invention is to make use of a desirable characteristic of the automatic volume control tube for accomplishing the control. This feature has the advantage of permitting very good control with a low potential on the plate of the control tube. This has been accom- ROL CIRCUIT e, Pa., assigner, by mesne Corporation of America,

Y., a corporation of Delaware 1932, Serial No. 637,681 (Cl. Z50-20) plished by supplying the proper potentials to the elements of the automatic volume control tube to cause it to operate on the desirable portion of its characteristic.

Another object of this invention is to control 6 the detector tube as well as the radio or intermediate frequency amplier tubes from the automatic volume control system. Since the detector has different operating characteristics than the amplifier this has been accomplished by control- 10 ling the detector and amplifier from different portions of the automatic control circuit.

Another object of this invention is to provide a connection for the actuation of the automatic control tube which does not unbalance the del5 tector stage, or prevent it from properly tracking with the other stages of the receiver. This has been accomplished by connecting the control element of the automatic control tube to the detector in such a Way that the capacity in the control circuit is not connected across the detector tuned circuit.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing wherein a preferred embodiment of one form of the present invention is clearly shown.

In the drawing:

Fig. 1 discloses a schematic diagram of a preferred form of the present invention. In this modification a radio frequency amplifier stage has been indicated as accompanying the control circuit, but it is understood that an intermediate frequency stage might be used or controlled instead of a radio frequency amplifier and still come within the scope of the invention.

Fig. 2 discloses a schematic diagram of a modification, showing the control potential taken from a different point of the control circuit than that shown in Fig. 1.

With particular reference to Fig. 1 of the drawing, I represents a radio frequency amplifier having a thermionic tube I0 which has a plate I2, a control grid I4, a cathode I6, an auxiliary grid I8, and the heater 20. As would be apparent to one skilled in the art, the input from a preceding stage would be connected to the grid I4 and cathode I6. The cathode I6 is grounded at 22. The auxiliary grid I8 is connected to a source of potential through a lead 26. The heater 20 is connected through its terminals 28 and 30 to a suitable source of potential. 'I'he plate'IZ is connected to one end of a choke 24, which at its other end,` 55

connects to a plate potential source through the lead 32. Condensers 34 and 36 are by-pass condensers and by-pass high frequency currents from the plate |2 and auxiliary grid |8, respectively, to ground at 22. Condenser 38 is a coupling condenser and furnishes capacity coupling between the circuit of the plate |2 and an inductance 40, through connections to the plate l2 and to one side of the inductance 40. Inductance 40 forms part of a tunable resonant circuit 46, composed of the inductance 40, a condenser 42 in series with the inductance 40, and a variable condenser 44 connected across the inductance 40 and condenser 42. One side of the condenser 42 is connected to the inductance 40 and the other side to a ground at 48, thus isolating the inductance 40 and preventing a D. C. potential from the ground 48 from being transferred through the inductance 40 to the input of a succeeding tube through a lead 50. The biasing potential for the succeeding tube is applied to thergrid of that tube thru the lead 50, inductance 40 and a suitable lead 4|. A condenser52 grounded on one side at 48 serves as an isolating and Vby-pass condenser to prevent D. C. potentials from the ground 48 from being applied to the input of a succeeding stage through a lead 54connected to its other side.

A detector |00 has a plate |02, a control grid |04, an auxiliary grid |06, a cathode |08, and a heater ||0 therein. The heater ||0 is connected to a suitable source of potential through its terminals ||2 and ||4. The input circuit connected between the grid |04 and cathode |03 includes a tunable resonant circuit |6 having an inductance ||8 connected at one end to the grid |04 and at its other. end to an isolating condenser |22. A variable condenser |20 is connected across the series combination of the inductance ||8 and the condenser |22 to tune the circuit to resonance at a desired frequency. The isolating condenser |22 is used in this stage to make it similar to the preceding stage so it will track therewith. Then, too, in a tuned radio frequency receiver, the rotor plates of the condenser |20 would ordinarily be grounded. However, in a superheterodyne receiver, the condenser |22 might not be used since in that case one side. of the condenser |20 would not ordinarily be grounded. A condenser |24 connected at one end to condenser |22 and at the other to the cathode |08 is also an isolating condenser, and while it also makes this stage similar to a preceding stage or stages, the two condensers |22 and 24 isolate the grid circuit from a cathode circuit so that a D. C. potential may be applied therebetween. AThe cathode |08 is connected to the negative -side of the power supply at a terminal |26 on a secondary |28of a power transformer through a lter choke |30 and resistors |32 and |34. Resistor |32 has a movable contact |36 connected to one of its ends and capable of short circuiting any desired portion of the resistor. A resistor |38 isconnected at one end to the inductance ||8, between that inductance and the condenser |22. The other end of resistor |38 connects to a terminal of, resistor |32 between that resistor and the choke |30. The cathode |08 is connected to ground at |42 through a choke |46 and a blocking condenser |82 and to a control electrode |44 of a control tube |40 through the choke |46. 'Ihe condensers |24 and 82 and the choke |46 form a radio frequency lter.

With particular reference to Fig. 2 of the drawing, the parts bearing the same numbers as those of Fig; 1 perform the same function, and are similar except perhaps for resistance values. However, in this modication, the lead |80 which supplies the control potential to the amplier grids is connected to a point 400 on a resistor |66a intermediate the ends of that resistor.

In some cases, it may be desirable to have a greater difference of potential between the anode 48 and auxiliary grid |50 of the control tube |40 than is desirable for a biasing potential for the controlled tubes. In this case, the modication shown in Fig. 2 might be` used. The point 400 on resistor |66a may be given a value such that the desired difference of potential may be obtained between that point and the anode |48. Thus, the potentials of the cathode |08, and the control electrode |44 are determined by the potential drop across the resistors |32 and |34, and may be manually varied by varying the position of the contact |36 on resistor |32. The value of resistor |34 determines the minimum potential. Resistor |38 allows the negative potential from terminal |26 through the choke |30 to be applied to the grid |04 and serves as an impedance to the ow of alternating current from the resonant circuit ||6. The choke |40 tends to prevent the alternating currents of the circuit of cathode |08 from being applied to the control electrode |44 of the control tube |40, but allows the D. C. potential of the cathode |08 tobe applied to the control electrode |44. The control tube |40 has, beside the control electrode |44, a plate |48, an auxiliary grid |50, a cathode |52, and a heater |54 therein. The heater |54 is connected toa suitable source of potential through its terminals |56 and |58 to heat the cathode |52. The auxiliary grid |50 is connected to ground at |60. The cathode |52 is grounded at |64 through a blocking condenser |62. The condenser |62 prevents the D. C. ground potential from being applied to the cathode |52. The plate |48 of the control tube |40 is connected, through a resistor |66 and a variable tap |68 on a resistor |12 to a voltage divider formed by resistors |10, |12, |14 and |16. Resistors |10, |12, |14 and |16 are connected in series, with a terminal of resistor |10 forming one extremity of the series connection, and grounded at |18, and a terminal of resistor |16 forming the other extremity and connected to one terminal of the choke |30 between that choke and the resistor |32. The potential of the cathode |52 of the tube |40 is provided by the connection of that cathode` to a point intermediate the resistors |14 and |16. The potential of the plate |48 of the tube |40 is supplied through the variable tap |68 on the resistor |12. The value of the resistor |12 with respect to the other resistors |10, |14 and |16 governs the limits of the potential variations on the plate |48. The plate |48 of the tube |40 is also connected to the grid I4 of tube I0 or grids of the amplier tubes which are to be controlled. As many amplifier tubes as may be desired may be controlled toy obtain the desired result. This connection is made through the lead |80. A bypass condenser |82 is connected between the plate |48 and ground at |18 to bypass high frequency currents from the plate circuit to ground. Hence, it may now be seen that any variations in the potential of the cathode |08 of the detector tube |00 will cause similar variations in the potential of the control eleotrode |44 of the tube |40, and consequently variations in the current flow between the plate |48 and cathode |52 of the tube |40.

The power supply utilizes the conventional in. The plates 202 areconnected tothe extremi' ties of the transformer secondary |28. The fllay ment 204 is connected across an auxiliary sec-V ondary coil 206 ofthe sametransformer, or a separate transformer secondary providing the proper voltage. The positive lead from the rectier which is connected to one side Vof the lament 204, is connected to one extremity of a voltage dividencomposed of resistors 2|0, 2|2, 2| 4 and 2|6 and high resistance choke 224, through a choke 208. A condenser 2|8 is connected across the choke 208 to form a part of the filter circuit. Condensers 220 and V222 are connected in series across the choke and their mid tap is connected to a lead from the terminal |26 of the coil |28. These co-ndensers form aV ny-pass for the ripple voltage. The lead from the tap |26, which is the negative lead, is connected to the other extremity of the voltage divider formed by the resistors '2|0, 2|2, 2|4 and 2|6, and the speaker field 224 which forms an additional choke and a part of the voltage divider. The point intermediate the resistor 2|6 and the speaker eld 224 is grounded at 226. A condenser 228 also connects to a point intermediate the resistor 2| 6 and the speakerV field 224 and to a point intermediate the resistor 2|0 and the choke 208, and forms an additional ripple by-pass circuit in the filter system. A condenser 230 is connected between the end of the choke |30 intermediate that choke and` the resistor |32, and the ground at 226. The potential of the auxiliary grid |06 of the detector |00 is supplied from a point intermediate the resistors 2 I4 and 2 I6. The potential for the amplier auxiliary grid and plate is supplied from a point intermediate the resistors 2 I2 and 2|4 and a point intermediate the resistors 2|0 and 2|2 through leads 232 and 234, respectively. The potential of the plate |02 of the detector v|00 is supplied from an extremity of the voltage divider intermediate the resistor 2|0 and the-choke 208 through an impedance 236 having a variable tap 238, the choke 240 and the choke 242. The condensers 244 and 246 connected tothe plate and the auxiliary grid, respectively, of the detector |00 and to the cathode through a common connection form by-pass circuits for high frequency currents. The Condensers 248 and 250 connected in series and across the choke 240 and grounded at their mid tap at 252 together with the chokes 240 and 242, form an output lter for the detector circuit. The choke 242 -is preferably a radio frequency choke, andthe choke 240 is preferably a high frequency audio frequency choke. The choke 240, however, may be omitted when desirable.

The tube 300 having anode 304, a grid 306 and a lament 308, and the tube 302 having an anode 3|4, a grid 3|2 and a filament 3|0 are connected in a push-pull audio frequency amplifier circuit. The grid 306 and a terminal of an input impedance 3|6 are connected through an isolating condenser 235 to the detector output filter at a point intermediate the choke 240 and the impedance 236. The grid 3|2 of the tube 302 is connected to the other extremity of the input impedance 3|6. The filaments 308 and 3|0 of the tubes 300 and 302 respectively are connected in parallel and supplied by a suitable heating potential through the leads 3|8 and 320. The ano-des 304 and 3|4 vofthe tubes 300 and 302 respectively are connected eachto an-.extremity of a primary 324 of an output transformer 322. The terminals of the secondary 326 of the output transformer 322 are connected across the terminals of the voice coil 328 of a loud speaker 330. The D. C. potential of the grids 306 and 3|2 is supplied from a point intermediate the resistors |14 and |16 through a resistor 334 and a tap 332 on the input impedance 3| 6. A con-denser 336 connected from a point intermediate the resistor 334 and the tap 332 to ground at |18 forms a by-pass circuit. The D.. C. potential on the anodes 304 and 3|4 of the tubes 300 and 302 is supplied from a point intermediate the resistor 2|0 and the choke 208 through a tap 338 on the primary 324 of the output transformer 322.

The impedance 236 has a movable contact 238 connected to one of its extremities and forming a variable contact intermediate its extremities so that a desired portion of the impedance may be short circuited. Variations in this impedance vary the impedance of the path of the plate current of the detector I 00 and the impe-dance of an audio frequency by-pass circuit.

Since one extremity of the voltage divider f-ormed by the resistors |10, |12, 14 and |16 is connected to the negative terminal |26 through the choke |30 and at its other extremity to ground, and since the auxiliary grid |50 of the control tube |40 is connected directly to ground, while the plate |48 of the control |40 is connected through a resistor l 66 to the point on the voltage divider intermediate the negative lead and ground it is apparent that the plate |48 is at a lower potential than the auxiliary grid |50. In fact, these potentials are so evaluated that the tube |40 acts on the desirable portion of its characteristic. Since the cathodes of the controlled amplifier tubes are at ground potential, and the grid of those tubes at the potential of the plate |48, it is also apparent that a negative bias is provided for those amplifier tubes. The fact that in this circuit the plate |48' may be operated at a lower potential than the auxiliary grid |50 shows clearly that the control tube can be any one of a number of varieties of tubes which operate with the auxiliary grid at a higher potential than the plate. This includes the pentodes and certain tetrodes which have their secondary emission limited.

Since the detector cathode |08 is connected to the negative lead through the resistors |34 and |32, that cathode will be above the potential of the negative lead by the potential equal to the Y drop of the resistors |34 and |32. Then, since the grid |04 of the detector is connected to the negative lead through the resistor |38 and the inductance H8, the potential of that grid wili be more negative than that of the cathode because under normal operating conditions no current will ow through the resistor |38, and hence there will be n-o drop in the potential across that resistor.

Variations in the strength of the signals received will cause variations in the current flow from the anode |02 to the cathode |08 of the detector |00. Those variations of current flow will vary the potential drop across the resist-ors |34 and |32, and consequently the potential of the cathode 08. Since the control electrode |44 of the control tube |40 is connected to the cathode |08 of the detector through the choke |66, the potential of that control electrode will be varied with that of the cathode. The result of the variations in the potential of the control electrode |44 will be to vary the current 110W between the potential of the plate |48 due to the drop in the resistors |66, |12 and |10. The potential of the grid of the controlle-d amplifier tube will vary simultaneously with that of the plate |48 due to their connection. In this manner, the gain or sensitivity of the amplifier tube is varied in response to variations in the strength of incoming signals.

Variations in the potential drop across resistors |34 and |32 due to variations in signal strength in the detector tube, while changing the potential of the cathode, do not appreciably change the potential of the grid |04. The variations in the relative potential of the grid |04 and cathode |08 amount to variations in bias of the detector tube and vary its gain or sensitivity. Variations in the value of resistor |32, by moving the variable contact |36 and short circuiting part of that resistor, control the possible minimum bias for the detector |00 and the control tube |40, thus controlling the current flow through those tubes. Variations of the movable contact |36 also controls the sensitivity of the high frequency ampliiier thru its effect on the control tube action since it varies the voltage applied to the control tube. This control is particularly desirable for compensating for the different modulation percentages of different stations. That is, it changes the action of the control tube so that the audio frequency output at the impedance 236 may be maintained practically constant for signals with either high or low percentages of modulation. It prevents overloading of the detector when signal-.s having high percentage modulation are received and permits the detector to be used at high efficiency when signals having low percentage modulation are received.

The control of the potential on the plate |48 of the control tube |40 through movement of the variable contacts |68 on resistor |12 also acts as a control of the current flow through the tube |40 and as a minimum bias control for the controlled amplifier tube. This control then controls the maximum sensitivity which may be reached by the amplier.

When a strong signal is received the current fiow through the detector |00 is high, causing the drop across resistors |34 and |32 to be high, which increases the bias on that tube and reduces its sensitivity. At that same time, the

.potential of the control electrode |44 is made more positive and the current flow through the tube` |40 correspondingly high. The high current iiow through the tube |40 causes a correspondingly large potential drop to cross the resistor in its place circuit, and thereby increases the bias on the amplifier tube by increasing the potential between its cathode or ground and its grid, thus reducing its sensitivity.

In a similar manner, when the signal applied to the detector tube |00 is Weak, the current flow through that tube is low, and the potential drop across resistors |34 and |32 is correspondingly low. This decreases the potential between the cathode |08 and grid |04, and increases the gain or sensitivity of a detector. At the same time the potential of the control electrode |44 is reduced to a less positive value and the current flow between the plate |48 and cathode |52 reduced. This decreases the potential between the plate |48 or grid I4 and ground, consequently increases the gain or sensitivity of the amplifier |0. Y i.

At times it may be convenient to reduce the audio frequency output for a given set of detector operating conditions. This may be done by reducing the impedance 236 by short circuiting a part of it through the movable contact 238.

While the form of embodiment of the present invention as herein disclosed, constitutes a preferred'form, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows:

l 1. In a radio receiver having a high frequency amplifier, a detector and a circuit therefor, an audio frequency amplifier, a reproducer, and a source of direct and alternating current voltages, the combination including, a control tube having a cathode, a control grid, an auxiliary grid, and a plate therein, a. direct current connection between the plate and the high frequency amplier input circuit, means coupling said control grid with the detector circuit, means includingV said source of direct current voltages for supplying a lower potential with respect to said cathode to the plate than to said auxiliary grid.

2. In a radio receiver having a high frequency amplifier, a detector having elements therein, and a reproducer, the combination including, a thermionic tube in said high frequency amplifier having a grid and cathode therein, an automatic gain control circuit, a thermionic control tube having a control grid, an auxiliary grid, and a plate therein, means maintaining said control tube control grid at substantially the same potential as one of the elements of said detector, means maintaining the amplifier grid at substantially the same potential as the control tube plate, and means maintaining the amplifier cathode at a potential substantially that of the control tube auxiliary grid.

3. In a radio receiver having a high frequency amplifier, a detector and a reproducer, the combination including, a thermionic tube in said high frequency amplifier having a grid and cathode therein, an automatic gain control circuit, a thermionic control tube in said circuit and having a plate and auxiliary grid therein, and means maintaining said amplifier grid at substantially the same potential as the plate of said control tube, said plate potential being approximately equal to or lower than the potential of said auxiliary grid whereby said gain control is accomplished with a relatively low potential on the control `tube plate.

4. In a radio receiver having a detector, an amplier, and an automatic gain control circuit, the combination comprising, means including said automatic gain control circuit for controlling the gain of said amplifier, means including a portion of said automatic gain control circuit disposed in the detector space current path for controlling the gain in said detector, means for predetermining the maximum gain in said detector, and said last means additionally being connected to the gain control circuit for predetermining the limit of the maximum gain attainable through the action of said automatic gain control circuit.

5. In a radio receiver having a detector including a grid, cathode and anode, an amplifier, and an automatic gain control circuit, the combination comprising, means including said automatic gain control circuit for controlling the gain of said amplifier, means comprising a variable resistor connected between the grid and cathode of the detector for predetermining the maximum gain in said detector, and said last means being connected to said gain control circuit for predetermining the limit of the maximum gain attainable through the action of said automatic gain control circuit.

6. An automatic volume control circuit for a radio receiver having thermionic tubes therein,

including in combination, a control tube having an anode, auxiliary grid, and control grid therein, said anode being connected with a grid of one of said therrnionic tubes, means for adjusting the direct current potential on said anode, Said auxiliary grid being at substantially the potential of the cathode of said one thermionic tube, and said control grid being connected with another of said thermionic tubes.

7. An automatic volume control circuit for a radio receiver, including in combination, a thermionio control tube having a plate, an input element, a cathode and an auxiliary element therein, means providing a lower direct current potential to said plate than to said other element, and means for varying the potential on said plate for controlling the maximum limit of sensitivity of said receiver.

8. In an automatic volume control circuit for a radio receiver, a thermionic control tube having an anode and an auxiliary grid and operating with the plate at a lower potential than that of said auxiliary grid, means for controlling the potential of one of said elements for adjusting Y one limit of the control action of said tube.

9. In an automatic volume control circuit for a radio receiver having a high frequency amplifier and a detector, the combination comprising, a thermionic control tube having a plate and auxiliary grid therein and operating with said plate at a lower potential than that of said auxiliary grid, and means including said tube for controlling the amplification of said amplifier.

l0. In a radio receiver of the type including a signal amplier Whose gain is to be controlled, a tube for controlling the gain of said amplifier, said tube including a cathode, an input electrode, an anode and an auxiliary cold electrode disposed adjacent said anode, means for impressing between the cathode and input electrode of said control tube signal-responsive energy whose amplitude variations control the variations in gain of said amplifier, a direct current connection between the anode of said tube and an input electrode of said amplier, and means for maintaining the anode of said control tube at a substantially less positive direct current potential than said auxiliary electrode whereby said gain control action is secured with a low potential on the said anode of the control tube.

` LAURENCE M. PERKINS. 

